Early personal computer, notable for including keyboard, monitor and tape drive into its all-in-one design.
-- ElliotMerrony

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Also well loved in scientific (laboratory) circles because of its IEEE 488 (HPGPIB) interface which allowed it to be connected to a wide variety of measuring equipment and related metering and control stuff.  -- GarryHamilton

I thought the PET had V 4.0 of Commodore Basic. V 4.0 was released before 3.5 with the C16/+4